Many types of biomass, such as harvest refuse, contain large amounts of energy, said energy being difficult to utilize because the energy density is low and it therefore being uneconomical to transport the refuse over any considerable distance. There have therefore been proposed combustion devices, the flue gases of which, being generated by the combustion of biomass, can be controlled as to volume and temperature to be of use for instance in drying processes. In this connection reference may be made to DE No. 1 451 508, DE No. 2 356 507, SE No. 345 900, SE No. 365 604, NO No. 120 887, and U.S. Pat. No. 4,159,000.
A specific problem in connection with the combustion of biomass of different kinds, such as harvest refuse, forestry refuse, etc., is the disposal of the ash. This problem is accentuated when the combustion is carried out at temperatures where the ash wholly or partly is fused or very soft, in other words forms slag, and is deposited on the walls of the combustion chamber. Since the temperature of softening and of fusion varies with different kinds of fuel, the character of the ash is also subject to variation, if the furnace is used for different types of fuel. The ash handling equipment must therefore be such as to accomodate both solid and more or less liquid ash products.
Among other things, this means that the equipment must be able to scrape off such ash products as are burnt to the walls of the furnace and to deliver them to a discharge opening, from where the ash may be discharged from the furnace. This in turn implies that the combustion process must be controlled in such a way that the ash products at least substantially are deposited within a predetermined area of the furnace. Further, the scraping means and the associated equipment must be located and designed in such a way that they do not disturb the combustion process or the currents of air and flue gases, or, if such disturbance occurs, that the disturbance is such that the combustion and/or the ash deposition are not considerably impaired. In addition, the scraping means and the associated equipment in themselves must be able to resist the strain which is inflicted upon them in the combustion area, and in case ash and slag are deposited on the scraping means, this shall not either impair their function.